Editing Entity–relationship model (section)

===Crow's foot notation===
{{See also|Cardinality (data modeling)|Process flow diagram}}
Crow's foot notation, the beginning of which dates back to an article by Gordon Everest (1976),<ref>G. Everest, "BASIC DATA STRUCTURE MODELS EXPLAINED WITH A COMMON EXAMPLE", in Computing Systems 1976, Proceedings Fifth Texas Conference on Computing Systems, Austin, TX, 1976 October 18–19, pages 39-46. (Long Beach, CA: IEEE Computer Society Publications Office).</ref> is used in [[Barker's notation]], [[Structured Systems Analysis and Design Method]] (SSADM), and [[information technology engineering]]. Crow's foot diagrams represent entities as boxes, and relationships as lines between the boxes. Different shapes at the ends of these lines represent the relative cardinality of the relationship.

Crow's foot notation was in use in [[International Computers Limited|ICL]] in 1978,<ref>"Introduction to Data Analysis", ICL Training Publication T2384 Issue 2, November 1978</ref> and was used in the consultancy practice [[CACI]]. Many of the consultants at CACI (including Richard Barker) came from ICL and subsequently moved to [[Oracle Corporation|Oracle]] UK, where they developed the early versions of Oracle's [[computer-aided software engineering|CASE]] tools, introducing the notation to a wider audience.

With this notation, relationships cannot have attributes. Where necessary, relationships are promoted to entities in their own right: for example, if it is necessary to capture where and when an artist performed a song, a new entity "performance" is introduced (with attributes reflecting the time and place), and the relationship of an artist to a song becomes an indirect relationship via the performance (artist-performs-performance, performance-features-song).

Three symbols are used to represent cardinality:
* the ''ring'' represents "zero"
* the ''dash'' represents "one"
* the ''crow's foot'' represents "many" or "infinite"

These symbols are used in pairs to represent the four types of cardinality that an entity may have in a relationship. The inner component of the notation represents the minimum, and the outer component represents the maximum.
* ''ring'' and ''dash'' → '''minimum zero, maximum one (optional)'''
* ''dash'' and ''dash'' → '''minimum one, maximum one (mandatory)'''
* ''ring'' and ''crow's foot'' → '''minimum zero, maximum many (optional)'''
* ''dash'' and ''crow's foot'' → '''minimum one, maximum many (mandatory)'''
<!-- 
These are notations of each side in the ER diagrams.

You can see an example of the Crow's Foot notation in the diagram above.
In the diagram, the following facts are detailed:
* An Artist can perform '''zero or many''' Songs
* A Song is performed by '''exactly one''' Artist
-->